1. Field of the Invention
The invention relates to an exhaust gas control apparatus and exhaust gas control method for an internal combustion engine.
2. Description of Related Art
It is known that a NOx purification catalyst is used as a technique for purifying exhaust gas emitted from an internal combustion engine, such as a gasoline engine and a diesel engine. However, precious metals generally used as a catalytic component of such a NOx purification catalyst, that is, for example, platinum group elements, such as rhodium (Rh), are increasingly used with tightening of emission regulations of automobiles and, therefore, exhaustion of resources is concerned. Therefore, it is necessary to reduce the usage of platinum group elements and to substitute other metals, or the like, for the platinum group elements in the future.
Then, there are many researches for reducing the usage of platinum group elements or for catalytic components as a substitute for the platinum group elements. Copper (Cu) is one of such catalytic components. Some suggestions about a NOx purification catalyst using copper and an exhaust gas control apparatus for an internal combustion engine, which includes such a NOx purification catalyst, are made (see, for example, Japanese Patent Application Publication No. 2001-003733 (JP 2001-003733 A)).
Generally, base metals, such as Cu, have a lower reducing ability than precious metals, such as Rh, in a lean atmosphere having excessive oxygen or in an atmosphere having near a stoichiometric air-fuel ratio. Therefore, it is difficult to sufficiently reduce and purify NOx contained in exhaust gas when the air-fuel ratio of the exhaust gas is lean or stoichiometric. Thus, for example, when Cu is used as the catalyst metal of a NOx purification catalyst, it is generally desirable to adjust the air-fuel ratio of exhaust gas to a rich atmosphere. However, Cu has a lower heat stability than precious metals, and the like, so, even when Cu is used in such a rich atmosphere, there is inconvenience that, as Cu is exposed to high temperatures for an extended period of time, Cu particles tend to agglomerate and form large particles. Then, Cu particles grown in this way cannot maintain a large contact area with exhaust gas any more and, as a result, the NOx conversion performance of the NOx purification catalyst decreases.